1. Field of the Invention
The present invention relates to an external additive for an electrostatically charged image developing toner which is used to develop an electrostatically charged image in electrophotography, electrostatic recording, and the like, and particularly it relates to an external additive for a toner with a small particle size used for obtaining images of high quality.
2. Description of the Prior Arts
Dry developers used in electrophotography and so forth are generally classified into a one-component developer which consists of a toner prepared by dispersing a coloring agent in a binding resin and a two-component developer consisting of the toner and a carrier. In using these developers in copying operation, the developers are required to have good flowability, anti-caking property, fixing property, electrification property, and cleaning properties so as to be adapted to the process. In order to improve especially the flowability, anti-caking property, fixing property, and cleaning properties, fine inorganic powder is frequently added to the toner.
However, the inorganic fine powder may greatly affect charging. For example, in the case of fine silica powder commonly used, it has so strong a negative polarity that it makes negatively chargeable toners too highly charged especially in a low-temperature and low-humidity environment, and on the other hand it takes up moisture to become low chargeable in a high-temperature and high-humidity environment, and hence the fine silica powder has such a problem that a great difference in chargeability may result between the both. As the result, it may make image density poorly reproducible and cause background fog. Also, the dispersibility of the inorganic fine particles may greatly affect toner properties. Non-uniform dispersion of a toner may not give any desired flowability or anti-caking property, or may result in insufficient cleaning property, causing adhesion of the toner on a photoreceptor and image defect in black spots.
For the purpose of making improvements on these points, the use of inorganic fine powders having been surface-treated to make its particle surfaces hydrophobic is proposed in variety. For example, Japanese Laid-open Publication (Kokai) Nos. 46-5782 (JP46-5782A), 48-47345 (JP48-47345A) and 48-47346 (JP48-47346A) disclose hydrophobic treatment of particle surfaces of fine silica powders. However, merely making use of the inorganic fine powders can not necessarily bring about any satisfactory effects.
Japanese Laid-open Publication (Kokai) Nos. 49-42354 (JP49-42354A) and 55-26518 (JP55-26518A) also disclose treating a powder of silica or the like with a silicone oil. However, a toner to which the surface-treated silica has been added may have low anti-offset properties to cause a problem that the toner adheres to heating rollers to contaminate the subsequent copies. This occurs because, when a wax added to the toner to impart releasability thereto and the fine silica powder treated with a silicone-oil become mixed, the wax builds up in viscosity to damage the releasing effect.
As methods of relaxing the strong negative chargeability of fine silica powder, it is known to treat particle surfaces of fine silica powder with an amino-modified silicone oil (Japanese Laid-open Publication (Kokai) No. 64-73354 (JP64-73354A)), to treat particle surfaces of fine silica powder with an aminosilane and/or an amino-modified silicone oil (Japanese Laid-open Publication (Kokai) No. 1-237561 (JP1-237561A)), to treat particle surfaces of fine silica powder with a quaternary ammonium salt (Japanese Laid-open Publication (Kokai) No. 5-100471 (JP5-100471A)), and to treat particle surfaces of fine silica powder with an amphoteric surface-active agent (Japanese Laid-open Publication (Kokai) No. 6-95426 (JP6-95426A)). Treatment with these compounds can keep the negatively chargeable toners from becoming too highly charged, but can not sufficiently lessen the environmental dependency inherent in the static electricity itself. In other words, it can somewhat keep the negatively chargeable toners from becoming too highly charged after their long-time use in a low-temperature and low-humidity environment, but can not still lessen the environmental dependency because such neutralization of electric charges may similarly occur also in long-time use in a high-temperature and high-humidity environment. Also, when a silicone oil is used as a treating agent, it has so high a viscosity as to cause aggregation of silica particles at the time of treatment, resulting in a poor powder flowability.
In addition, when an organic photoreceptor or a toner with a smaller particle size is used to improve image quality, the use of the inorganic fine powder does not give sufficient performance. The organic photoreceptor has a softer surfaces and a higher reactivity than an inorganic photoreceptor; therefore, the life of the organic photoreceptor is liable to become shorter. Such an organic photoreceptor is liable to change in quality or to be abraded at its surface because of the inorganic fine powder added to the toner. When a toner with a smaller particle size is used, the toner is low in flowability as compared to toners with a conventional particle size. Therefore, the inorganic powder has to be used in a large quantity, and thereby the inorganic fine powder may have caused the toner to adhere to the photoreceptor.